Since expanded resin beads are excellent in lightness in weight, cushioning properties, and heat resistance and high in a degree of freedom of shape design with which three-dimensional molded articles are obtained by means of in-mold molding, they are utilized as a cushioning material, a container, a heat insulating material, a vibration-damping material, or the like in multipurpose fields inclusive of packaging fields, commodities for living, building and civil engineering materials, vehicle members, and the like. The expanded resin beads are roughly classified into those made of a styrene-based resin as a base material resin and those made of an olefin-based resin as a base material resin. Among them, the expanded resin beads made of a styrene-based resin as a base material resin are used more frequently than the expanded resin beads made of an olefin-based resin as a base material resin for the reasons that the former is excellent in a balance between lightness in weight and compression strength, is easy for in-mold molding, and is inexpensive. But, though the expanded beads made of a styrene-based resin have an excellent aspect as described above, they are insufficient in heat resistance, chemical resistance, durability, toughness, and creep resistance characteristic depending upon uses.
For that reason, among the olefin-based resins, the expanded beads made of a propylene-based resin, which are excellent in heat resistance, chemical resistance, durability, toughness, creep resistance characteristic, and so on, are watched. But, with respect to in-mold molding using expanded propylene resin beads, there is involved difficulty in the aspect of molding processing, for example, a molding pressure of a heating medium, such as steam, etc., is high due to crystallinity or heat resistance of the propylene-based resin, and hence, an improvement is required.
For example, PTL 1 describes that in order to utilize fusion characteristics by a propylene-based resin having a low melting point, thereby decreasing a molding pressure of steam at the time of in-mold molding, while revealing appearance, heat resistance, and mechanical physical properties of a propylene-based resin having a high melting point, the propylene-based resin having a high melting point and the propylene-based resin having a low melting point are mixed under specified conditions.
In addition, for example, PTLs 2 and 3 disclose that in order to decrease a molding pressure of steam at the time of in-mold molding of expanded propylene resin beads, expanded propylene resin beads in which a core layer thereof is covered with a resin having a low melting point are used.